Email and authentication services for up to 135,000 staff working in schools across New South Wales are expected to be back up and running tomorrow, the Department of Education and Communities has said.

"Today, there has been an issue with school staff access to some computer services, including email," a department spokesperson said. "The department has been working to resolve this as a matter of high priority, and it is hoped services will be returned to normal by the start of business tomorrow."

The spokesperson said that school teachers are still able to access the internet and locally held resources.

The cause of the outage of email and authentication services, which are believed to have been down since the middle of Monday, is not yet known at this stage, but the department is investigating the cause, the spokesperson said.